Upgrading mixing operations and adding mixing capacity to an existing mill room.
As with any project, the starting point for undertaking the upgrade or addition of capacity to an existing mixing operation is the development of a clear statement of objectives and a means of determining whether or not the work has met or exceeded those objectives. The development of those objectives, the definition of what is to be accomplished, why and how, is formulated from the answers to the following questions:
* What is the present situation: What business is the company in; how is that business changing; what are the capabilities of the present facility; what assumptions underlie the business forecast and/or opportunities?
* What is the opportunity or need that an upgrade or capacity addition will address? What is the most cost-effective way to achieve those objectives?
* Can the opportunity or need be met by improving the productivity of the existing facility, or is an additional mixing line required?
Improving utilization of the existing plant and equipment
Improvement in productivity is an increase in total output relative to the consumption of a resource. The utilization of any resource is improved when the ratio of useable output to total output is increased. When there is an increase in the percentage of output that meets specifications, we have reduced the resource consumption required to produce a unit of usable output.
Productivity can be increased by decreasing the quantity of an input required to produce a unit of output; providing we can do so without incurring a cost that exceeds that gain: If we reduce one input by incurring an increase in the use of another input, the cost of the reduction in one input must not be offset or exceeded by the increase in cost of other inputs.
In pragmatic terms, the available inputs in an existing mill room are space, energy, labor and time. Time here being a form of capital, it is a measure of the efficiency of the utilization of the investment in plant and equipment. The initial point of attack for increasing productivity is time, how much usable output is being realized per unit time from the fixed investment in plant and equipment. In order to begin looking for opportunities to improve output per unit time, accurate information on the present utilization of the available time is essential.
The collection of data and the verification of that data are the first and perhaps the most important steps in improving productivity. For a given line, we need to know when the line was producing usable product and at what rate, and if below design rate, why. When the line is not producing usable output, we need to know why, and we need to be sure that the "why" reported is the actual reason, not simply an assigned reason.
Often, a careful analysis of the data collected over a reasonably long period of time produces some surprising results. As an example, a four-line plant with a mix of continuous and batch compounding equipment producing a variety of fairly technical formulations was engaged in the preliminary engineering work for the addition of a new mixing line. In the process of collecting information to support that work, it became apparent that almost 50% of the available production time was spent cleaning the post mixer extrusion and pelletizing equipment when a formulation change was made. Additional production capacity equal to what would be achieved by adding the new line was available by taking steps to improve the ease of clean-out and changeover for very little capital investment.
On another occasion, a few years earlier, a plant engaged in a study with the objective of identifying additional equipment that might improve the efficiency of the batch mixer; more pounds per hour of compound were required. During the course of collecting data to determine what might be done, where things could go, whether the necessary power was available, and in what manner the changes might affect current operations, it was observed that during a production run, the mixer was routinely empty between 30-40% of the time. The labor agreement required the mill operator to handle only so many batches per hour, so the mixer output was slowed so as not to exceed that number. In this case, adding equipment and changing some aspect of the process were necessary steps to negotiating a work rule change, liberating some of the already-in-place capacity.
As these examples illustrate, it is often the case that the limiting factor in mixing line productivity is not the capacity of the mixer, nor even the capacity of the post-mixer finishing equipment, whether that might be a mill, roller head extruder or strainer, but rather the way in which the work is scheduled and the time necessary to prepare the equipment for a product change. The identification of these issues is essential in order to avoid unnecessary or ineffective equipment changes or additions. The potential results from making improvements or corrections in this area often require minimal capital investment. Should further improvement in productivity or capacity still be necessary, the information gained permits making informed decisions in selecting from among the options for doing so.
De-bottlenecking pre-mixer and post-mixer operations
The data collection and analysis discussed above provides the information necessary to proceed to the next step--determining the constraints in the existing mixing line, and evaluating the benefits and costs associated with relieving those constraints in the jargon of the process industries, de-bottlenecking the line. Setting aside the mixer itself for the moment, there are two areas of opportunity: The delivery of material to the mixing line, and the capacity and/or efficiency of the post-mixer finishing and cooling equipment. It makes little sense to improve the throughput capacity of the mixer if that capacity would then exceed the capability of the supporting feeding, weighing and post-mixer cooling and handling equipment.
Taking the post-mixer equipment first: Except for custom and toll compounders, the output of the mixing operation is an intermediate step in the manufacturing of a product. The requirements of the next step in that process dictate the form required of the output of the mixing line, wig-wagged slab on pallets, cut and stacked slab, strip wig-wagged into boxes and strip wound on "hats" (strained or not, as required). Chemistry and process dictate acceptable targets for the mid-plane temperature of the stock as it moves from the mixing line to in-process inventory or shipment. If the stock is strained, there are similar limitations as to how hot the stock can get as it passes through the strainer. Each element of the post-mixer line has some throughput limitation. Identifying that limiting condition or factor, and determining how it might be modified to increase the capacity, is the next step in the process. Assuming space is available, the length of a batch-off can be increased; a batch-off could be added, if the mill is being unloaded manually; if contact with water is acceptable, a spray cooling section on the front end will decrease cooling time; elevating a portion of the batch-off or cooling conveyor may make enough space available to increase its effective length.
Replacing an older strainer with a newer (not necessarily new) one with a larger screen area and better heat transfer capabilities will increase flow rate; mechanized stacking/wigwag, or pallet handling systems may reduce or minimize delays. Rebuilding an existing strainer to increase its capacity (more robust thrust bearing assembly, overhauling and/or reengineering the head locking and support system to permit higher pressures, as examples) may offer a lower cost way of relieving a constraint.
It may be appropriate to look at different methods of converting the mixer output to a strip or slab. For many compounders, the open two-roll mill is the installed device, and the one that is best understood and with which they are most comfortable. Of the available post-mixer devices, it is the most flexible, and offers the ability to perform additional mixing and blending operations in tandem with the intensive internal mixer that precedes it. In addition, a mill remains the easiest of all of the post mixer devices to clean, all of the surfaces that come into contact with product are readily accessible. It product contamination concerns warrant, fixed stock guides (also called "end dams") can be replaced with air-lift or tilting stock guides to facilitate clean-up under the guides.
Other alternatives are fundamentally variations on a theme, some form of mixer dump extruder, with a variety of discharge options. All offer the ability to convert the mixer discharge to a slab without operator intervention. The now fairly common tapered twin-screw with roller head discharge is a low back-pressure slab forming device with the ability to accept a mixer batch without a feeding assist device. Older mixer dump extruders were usually single screw extruders, most fitted with a robust reciprocating "pusher" of one type or another to stuff the batch into the feed throat area of the screw. These units could be fitted with discharge devices of various configurations, depending on the process application, and, with appropriately selected thrust bearings and locking mechanisms, develop considerable pressure on the batch. An example of such a machine is the variant developed for certain tire applications that employed a pair of pivoted rolls. The design offered the ability to develop relatively high nip pressure while permitting quick clean out of the area between the nose of the screw and the roll nip.
Shifting from a drop mill to a dump extruder of any variant is usually done for process, safety or manpower reduction considerations rather than as a means of increasing throughput. It is rare for the drop mill to hold up the mixer. With few exceptions, a post-mixer extruder does no "finishing" of the mix; it is not a very efficient cooling or heat transfer device; it does eliminate the need for an operator, which decreases accident exposure, training requirements and the possibility of operator introduced variability in immediate post-mixer operations.
A mixer dump extruder, properly sized and matched to the mixer, can transform a batch mixing operation into a continuous process, an advantage for multi-batch production runs, a disadvantage for processes that require batch traceablilty. If properly configured, it may produce a more consolidated slab more rapidly than a mill would. Fitted with a two-roll vertical calender at the discharge, the dump extruder can directly convert mixer discharge to heavy gauge void-free sheeting in widths up to several feet, perhaps eliminating one or more calendering operations in product-specific mill-room operations.
Mixer batch time has three components: Feeding, mixing and discharge. Mixing time is usually defined as the time between initial ram/weight down to ram/weight up prior to discharge; discharge time is discharge/drop door open to discharge/drop door closed; the feeding portion of the cycle is that period between discharge/drop door closed and ram/ weight down.
To digress briefly: When the ram or weight is initially lowered, there is usually a short period of time before it seats. During this period, the dry ingredients are folded into the polymer and deaerated. If the batch has been properly sized, the ram will move downward as this occurs. When it ceases moving downward and begins to move up and down slightly just above its full down position, it is seated, and effective mixing is underway.
I will address factors affecting mixing efficiency in the next section. What is of interest in this section is length of time required to introduce material to the mixing chamber. Here again, observation and measurement are essential in uncovering opportunities for improving utilization and productivity. One of the attractive features of a batch mixer for short run applications and start-up operations is that is does not require any form of mechanized or automatic feeding and/or weighing equipment. All materials can be manually weighed and manually charged to the mixer. Very little preparation is required for most batch mixers; cutting of bales is necessary only to produce the required weight per batch. The mixer is capable of accepting and digesting full bales.
How materials are weighed, transported to and charged to the mixer is determined by a complex matrix of concerns: Minimizing the time required to both introduce the material to the mixer and get to the ram/weight seated point in the mixing cycle; how to insure that weighments are made accurately and consistently enough to insure that product will both meet specification and not require excessive use of costly ingredients to insure that those specifications are met; that properly weighed or measured ingredients are fed to the mixer in a manner that insures that quantities so weighed or measured are incorporated in the batch and not lost to the operating environment; minimizing the cost of labor required to carry out the necessary steps in the feeding and weighing process; maximizing the return on any capital invested in equipment employed to mechanize or automate the feeding and weighing process; balancing material preparation costs and time against batch cycle time (e.g., time and energy spent reducing rubber bales to smaller, warmer pieces against time spent in the mixer); and balancing feeding and weighing costs (labor, equipment, material loss, mixer utilization) against process documentation and verification requirements.
For every mixing operation there is a unique matrix of requirements that will dictate the approach to each of these questions. For most installations, that matrix can be expected to shift over time, and allowances need to be made for those shifts. For a captive mixing facility supplying compounded material for a closed cell foam insulation plant, the number of formulations and the number of raw materials are limited. For a captive mixing facility supplying compound for rubber-to-metal seals and composite springs and mounts, a very large number of formulations and ingredients may be involved. For the production of rubber seals bonded to metal inserts mixed with the molding of unsupported seals, the number of ingredients and formulations might be extremely high, and coupled with the requirement to document the formulation and the process.
Some overall principles do apply:
* Reduce the time required to charge the mixer;
* minimize the possibility of error;
* reduce the labor input per pound of production;
* reduce the possibility of injury; and
* reduce the energy required to process the material.
* Mechanize the feeding of dry powdered or granular materials where possible, and avoid charging them through the open mixer charging door. Handle these raw materials in bulk or in semi-bulk containers where recipe utilization permits. These techniques reduce material loss, improve the environment around the mixer, reduce manual labor and decrease the time required to transfer material into the mixing chamber.
* Avoid charging liquid additives through the mixer charging door. Instead, inject the liquid additives into the mixing chamber through one or more liquid injection nozzles. Properly designed and maintained, these systems help insure that the liquid additive all gets into the chamber, and avoids coating the charging door surface with sticky substances that impede the adequate charging of other ingredients. Liquid injection systems can run from the very simple to the fully automated, depending on the number of liquids to be handled. The simplest employ hand volume measurements; a container filled to the proper level is dumped into a holding tank for injection at the appropriate point in the mixing cycle. More complex systems may be based on volumetric, net-weight or loss-in-weight gravimetric systems, or a combination of techniques and may provide for splitting the liquid addition into two or more separate injections, depending on the mixing requirements and the ability of the batch to accept the liquid component.
* Consolidate the minor additive additions into one or more melt-compatible bags. This technique insures that all of the additive package gets into the mixing chamber; facilitates a variety of techniques for confirming the addition of the proper additive package, and reduces charging time at the point of introduction to the mixer.
* For smaller volume mixers, in the 80 liter and less range, consider reducing baled polymer additions to chunks in the 4-6 kilogram range. This will decrease the initial breakdown time and shorten the elapsed time from ram/weight down to ram/weight seated.
The emphasis up to this point has been on those activities related to getting material into the mixer, primarily because our focus is on improving the productivity of the mixing function. All aspects of raw material handling are candidates for productivity improvement. Some have a direct impact, for example, avoiding the deterioration of material while in storage or handling; maintaining lot and ingredient identity; controlling inventory flow for consistent performance at the mixer, in the compounded material and in the product. Other aspects impact the overall facility productivity, control of loss or spoilage, efficient utilization of storage space, organization of material flow for economy of transport, and balancing the cost of material in inventory against unit cost of purchasing.
Increasing usable mixer output
Having fairly quickly summarized the areas for potential improvement before and after the mixer, and having concluded that we have optimized those areas from a cost-benefit standpoint, we now move to the mixer itself. We can make gains in productivity by reducing the time it takes to produce a batch of acceptable quality and specification, and we can also improve productivity by reducing or eliminating the number of batches that are rejected, scheduled for rework, require extensive finishing or must be sold at a lower price as off-spec material.
Assuming that the mixer is in good repair, the low-cost opportunity is optimization of mixing parameters. Plants which have been functioning for any period of time acquire patterns of equipment operation that become standard. Hidden inefficiencies accrue as practices learned to deal with particular problems become the norm for all situations. Mixers wear over time, resulting in lower shear intensity at rotor tips and an accompanying effective increase in chamber capacity; formulae change, raw materials allegedly equal in specification in fact undergo subtle changes in processing characteristics when suppliers are changed; auxiliary systems become inoperative, unreliable or judged too bothersome to take advantage. A disciplined approach to optimizing batch sizes and operating conditions will often result in an improvement in machine productivity, larger batches on shorter cycles with improved consistency and tighter quality bands.
Equipment which is excessively worn introduces limitations and inefficiencies best overcome by repair or overhaul. Worn rotors and/or mixer bodies reduce mixing efficiency, increasing the time necessary to achieve the required level of dispersive and distributive mixing and/or the necessary level of viscosity reduction. Worn dust stops may limit ram and batch pressures, resulting in a slower than optimum rate of temperature rise; excessive wear in the weight will result in raw materials losses to the ventilation system and the operating environment, as well as variation in product quality; and fouled water circulation channels in the mixer sides will result in less than optimum heat transfer and unnecessarily lengthy mixing cycles. Plant air systems which are not in good operating condition or are not sized to support the consumption rate of the mixer can result in inconsistent or extended mixing cycles and slower mechanical cycling of the mixer.
A mixer body which has become worn to the point of needing replacement provides an opportunity to consider a change in the mixer technology. For many years, the standard mixer rotor for a non-intermeshing intensive internal batch mixer was a two-wing design running at a friction ratio, and mixer chamber and rotor cooling were pretty much all or nothing propositions. Over the last 40 years, there has been a steady evolution and proliferation of rotor configurations. Mixer manufacturers have worked with major clients to optimize rotor designs to achieve tailored objectives and overcome observed shortcomings in earlier designs. Cooling has evolved into temperature control, sometimes called tempering, of mixer chamber sides, rotors and door top. Mixer chamber sides have seen significant improvements in heat transfer capabilities, steady improvement has been made in the heat transfer in the characteristics of rotors, and heat transfer capabilities have been extended to rotor end plates as well.
[FIGURE 1 OMITTED]
The following is specifically related to the replacement of a worn mixer body for a tangential rotor intensive internal mixer. There are a number of things to consider: First, perhaps, is chamber volume. Although the number of 11D and 3D mixer installations has been declining over the last three decades, their number remains significant. (The "D" designation referred to "drop" door discharge door design. Earlier mixer designs employed a sliding door that retracted along the same axis as the mixer rotors toward the water end of the mixer.) When mixer manufacturers upgraded mixer designs, the 270 liter and 80 liter sizes were designed with the same rotor center distances as the 11D and 3D mixers, to permit making use of existing drive trains, feed hopper assemblies and in some instances, bedplates. The mixer chamber volume is in each case larger, and much depends on rotor selection. More recently, Farrel has introduced two new sizes of mixer that replace their forerunners in the same manner: The F-305 is a direct replacement for the F-270, and the F-440 will replace the F-370. (The Farrel F Series mixer designations were intended to be the water level capacity of the mixing chamber. Both the F-370 and the F-620 designations significantly understated the chamber size. The F-370 has a chamber capacity with four-wing rotors of 414 liters; the F-620 has a capacity of 710 liters. The new F-305 is nominally a 285 liter machine; the new F-440 is listed as having a capacity of 338 liters. As discussed in the text, actual capacity depends on the selection of rotor type, and to a lesser extent, on the floating weight or ram configuration.)
Other considerations include rotor type, type of side, provision for liquid injection nozzles, location and type of temperature sensing element, type of dust stop and internal finish. When considering any of these changes, the objectives defined at the outset are to be kept in mind. Changes in mixer body specification may reduce mixing time, might improve the quality of the mix, or perhaps both, but they will change present operating procedures. Even if a proposed change in rotor type has been evaluated in a demonstration facility, there will still be a period of trial and adjustment once the new mixer body is installed in the plant. For a given rotor speed, more aggressive rotor designs will increase the rate of work input per unit time, normally at some sacrifice in available mixer chamber capacity. The quality of dispersive and distributive mixing may or may not be enhanced by the new rotor design. Improved heat transfer capability in new generation rotors and mixer sides may require adjustments to the temperature of the circulating heat transfer fluid for optimum mixing performance. Simply replacing a worn mixer body with a rebuilt or new body may require adjusting batch size due to the reduction in working volume.
The foregoing changes all assume that a body change alone is contemplated. The three other major components of the mixer are the drive train, the feed hopper and the bedplate/discharge door/latch assembly.
Mixers with slide door discharge openings are relatively uncommon in the present inventory. A few remain in applications where the characteristics of the design are favorable for the process. (A slide door is basically a large air cylinder with a fixed rod and moveable cylinder. A "door" shaped to fit the open space between the two mixer sides is attached to the top of the cylinder, parallel to the axis of the cylinder. A step of stepped gibs causes the door to "step up" against the sides and end plates as the door closes. When the batch is complete, the cylinder is retracted, dragging the door past the end flame of the mixer body. The sliding action of the door scrapes sticky products off the door top as the door retracts. Where this characteristic is not an advantage, it tends to produce "rat tails" as the door retracts, which frequently hang up between the mixer and the mill, and become a cross-batch contamination problem and a house-keeping issue. The fit-up of the door to the body is crucial both to properly seal the opening at the bottom of the chamber and to minimize the production of "rat tails.") Where this is not the case, the door is relatively slow acting, requires attention to keep it properly adjusted, and is prone to producing "rat tails" as the door retracts, an housekeeping issue and possible source of cross-batch contamination. The drop door design opens and closes quickly, and is designed with a "floating" door top that is to a large extent self-adjusting to compensate for wear. Existing slide door mixer installations can be upgraded with drop-door bodies; the change usually entails some modifications to the bedplate, but permits making use of the existing drive train and feed hopper, and can be accomplished relatively quickly.
[FIGURE 2 OMITTED]
Mixer feed hopper design has seen several changes over the years. As the trend away from manual charging of all ingredients through the front hopper door to more mechanized and automated feeding systems took hold, over time these changes became the new standard. The angle of the hopper door was changed; one finds reference in older literature to the introduction of the "kwik-feed" hopper design; hopper extensions, variously called "fluff necks" or "distance pieces" were added between the bottom of the hopper and the top of the mixing chamber to provide a place for dry ingredients to deaerate after being charged to the mixer; hoppers were provided with a provision in the side and backplate for the ready installation of weigh hopper discharge chutes; hoppers became standardized with integral distance pieces; and renewable wear plates in the hopper throat were added to the design, in some instances extending into the mixing chamber. Variations in the design of the floating weight were evolved, ram/weight cylinder bores were increased and hydraulically actuated rams offered; variable ram pressure controls fielded; and ram position sensing systems and differing hopper door actuating methods employed.
The ram or floating weight is a key player in mixer productivity and proper mixing operation. For a given rotor speed, varying ram pressure varies the rate of energy absorption by the batch; as ram pressure increases, the rate of batch temperature increase rises. In general terms, when the batch is first charged, a low ram pressure is necessary to avoid pushing the dry ingredients up around the weight clearances and perhaps through the dust seals. Once the dry ingredients have been incorporated in the batch, a higher ram pressure can be put on the batch, decreasing the free volume in the mixing chamber. Adding variable ram pressure controls to an existing hopper weight cylinder is a relatively easy and inexpensive task. If, however, the hopper is an older design with a small weight cylinder, upgrading the hopper to one with a larger weight cylinder may enable an improvement in mixing cycle time. Replacing an older hopper assembly with a newer one may permit more rapid and reliable mechanically assisted feeding of natural or synthetic polymers to the mixer; replacing older mixer hoppers with a no fluff neck or distance piece with newer designs having an integral distance piece, or adding a distance piece to an older hopper can reduce the time required both to feed and incorporate low bulk density dry additives. (This requires a change to the weight cylinder and rod as well; they must be longer by the height of the distance piece).
[FIGURE 3 OMITTED]
There are advantages to the various designs of floating weight, and it is worth reviewing the arguments for each in light of the plant's current operating experience and planned product mix.
First introduced three or four decades back, hydraulically actuated hopper cylinders have once again become attractive in certain applications. The main driving force seems to be the cost of compressed air; a process advantage is claimed in certain applications due to the stiffer nature of a hydraulic cylinder in comparison with an air-loaded cylinder, and the closed hydraulic system permits consistent and repeatable ram pressures.
Drive train improvements and/or changes can be made independently or in conjunction with other mixer upgrades. Generally, the objective is to increase the mixer rpm, increase the connected torque to deal with stiffer compounds, or to replace badly worn gearing. A somewhat less costly upgrade in more recent times involves the conversion of rotor drive from friction to even speed.
A little history on mixer drive trains is helpful in understanding what is in the installed base, what is available on the previously owned market, and what some of the terminology is intended to describe.
Older tangential rotor mixers were designed to operate with a speed difference or friction ratio between the two rotors, hence the terms fast rotor and slow rotor. As with two-roll mills, this ratio was established by means of connecting gears mounted to rotor extensions outboard of the main bearings. One of the two rotors, called, of course, the long rotor, extended even farther out from the mixer body. In early drive arrangements, a bull gear was mounted on this extension and driven by a pinion gear. In very early mixers, the pinion gear was carried on a jack or pinion shaft, and coupled directly to a large low rpm motor. Later on, higher rpm motors drove a single reduction gearbox with an extended low speed output shaft carrying the pinion. The next evolution in the design of the drive train saw all the reduction gearing in a single reducer; this was sometimes termed a semi-unidrive. The next step was to take the connecting gearing off the rotor necks and put it into a gear box, now called a unidrive. Unidrives came in two basic ratings, standard and heavy-duty. There are, of course, almost endless variations and combinations. Some smaller mixers built for service on PVC were fitted with right angle worm gear reducers, which made for a very compact arrangement, especially useful when two mixers needed to be placed relatively close together. In some installations, the bull gear was carried on an independent shaft supported in pillow blocks and flexibly connected to the "long rotor."
Recent work by mixer manufacturers of tangential rotor machines has demonstrated that "even" or "synchronous" rotor speed is advantageous with certain rotor designs. This claim has demonstrated merit, but the corollary to it is that although the orientation of the rotors with respect to each other is significant, it is not enough to simply replace friction gearing with even speed gearing. Replacing connecting gearing is relatively simple on mixers with open connecting gearing mounted to the rotors. It is not terribly difficult for mixers with fully enclosed gearing (unidrive gear boxes), but it is more time consuming, and will require mixer down time in excess of the normal mixer body changeover, no matter how well planned.
There are probably several approaches to the work. Perhaps the most straightforward is to work with the original gear reducer builder, as the dimensional information needed to fabricate the necessary replacement components will be in their records. An alternate approach is schedule the work with a gear repair facility. In this instance, the reducer would be partially disassembled and component parts to be reworked shipped to the repair facility. The needed dimensions and details of assembly can then be taken from components and new gearing manufactured to suit.
When increasing speed or torque and working with an existing drive train, it is necessary to determine the initial design torque rating of the drive. For our purposes, we can define the drive train design torque in terms of the horsepower per rpm of the fast rotor. Generally, for most manufacturers, this design was based on a service factor of 1.7 to 1.8: that is, the catalog or design rating was 1.7 to 1.8 times the connected horsepower when delivered to the first user. In many instances, the original drive train has been modified over time, so care and research are essential. Reference to older catalogs, sales literature, or in some cases installation drawings, will reveal that open connecting gearing had an upper horsepower per RPM design limit; in some instances, in the absence of complete name plate data, older catalogs will provide information on the design horsepower rating of gear reducers. (Exercise caution, and verify the ratio of older boxes. Older boxes have occasionally had the gearing changed, with no change made to the reducer name plate data.)
Care must be taken not to increase the design applied torque when making changes to the drive train. As an example, take a mixer with a fast rotor speed of 20 rpm, designed for and presently connected to a 600 rpm 600 horsepower motor. For process and productivity reasons, we want to double the rotor speed to 40 rpm. Replacing the existing motor with a 1,200 rpm 1,200 horsepower motor delivers the target speed at the original design torque, 30 horsepower per rpm. On the other hand, the seemingly innocuous project to cut the mixer top speed in hall, accomplished by interposing a 2:1 reduction between motor and reducer with a belt and pulley drive, doubles the connected torque. Assuming the process demands that torque, there is a very high risk of damaging or even destroying the gear train.
(This is the first part of a two-part article. Part II will appear in the August issue.)
by Lawrence R. Gooch, Gooch Engineering Associates